1. Field of the Invention
The present invention relates to an optical module.
2. Description of the Related Art
A CAN-type optical module (also referred to as “optical subassembly” (OSA)) used in optical communications includes a CAN-type optical package in which an optical semiconductor device that performs photoelectric conversion is mounted, and a flexible board for electrically connecting the CAN-type optical package with a main board (JP 2009-302438 A and JP 2007-287767 A). On the flexible board, a pair of differential transmission lines are provided close together so that the pair of differential transmission lines are electromagnetically coupled together to obtain desired characteristic impedance.
The flexible board is extracted so as to protrude from the CAN-type optical package. By providing a GND lead pin at a position serving as a bend stop in the protruding portion, the bending of the flexible board can be prevented to thereby prevent disconnection of a wiring such as the differential transmission line. That is, the GND lead pin is provided at a position close to the protruding portion of the flexible board in a region where the flexible board overlaps the CAN-type optical package.
However, when the GND lead pin is provided at this position, the pair of differential transmission lines are away from each other with the GND lead pin interposed therebetween, and each of the pair of differential transmission lines becomes an independent transmission line. For example, the line width of the transmission line needs to be changed to match impedance; however, such a change point constitutes a factor for impedance mismatch, finally leading conceivably to adverse effects on characteristics. Therefore, the pair of differential transmission lines are desirably routed to connection terminals in the state where the pair of differential transmission lines are close together as much as possible.